1. Field of the Invention
This invention relates to fail-safe spring brake actuators for heavy-duty motor vehicles. More specifically, it relates to an improved air-released, spring-applied brake actuator which provides more actuator stroke per unit of spring brake chamber length and chamber weight, has desirable safety features, has fewer components and simplified construction and can be manufactured at competitive cost.
2. Discussion of the Prior Art
Heavy duty vehicles such as trucks, particularly when loaded, have high inertial masses which require powerful braking systems for control at the high speeds common today. Such systems are usually fluid operated and controlled, typically employing pressurized air as the fluid medium.
Since air pressure may be lost due to a number of common exigencies, e.g., compressor failure, hose rupture, connector problems, miscellaneous leakage in the system and the like, the service brakes require a fail-safe backup which automatically applies the vehicle brakes in the event of significant air pressure loss. Similarly, a fail-safe backup is dictated for the parking mode, particularly in view of long-term parking exigencies. A popular backup system relies on a powerful coil spring controlled by pressurized air within a chamber and operatively connected to the brake actuator arm so as to release the brake in the presence of adequate air pressure and to apply the brake in the absence thereof.
These air-released, spring-applied assemblies are well known and the chambers for same are advantageously integrated with service brake chambers, a widely-accepted application of the technology in the heavy-duty motor vehicular field. Accordingly, while the present invention is directed primarily to the spring brake assembly, it will be illustrated herein in connection with the combination spring brake and service brake integrated assemblies, although not necessarily limited thereto.
The assemblies illustrated in many prior art patents and other prior art suffer from certain limitations, the solution to certain of which the present invention is directed. Thus, for example, one prior art limitation is the insufficiency of the brake actuator stroke. Manifestly, such deficiency limits brake performance or duration thereof, requires more frequent actuator adjustments and lining replacements and is otherwise undesirable, as those skilled in the art have recognized.
While actuator chambers can be readily designed or re-designed and enlarged to provide a longer stroke, and even assuming the associated increased costs are acceptable, the space available for installation of the brake actuator may be limited, thereby preventing such enlargement. Moreover, even if space is not limiting, the typical combination spring-brake, service-brake combination is mounted on the truck or trailer underside by means of several bolts at one extremity of the brake chamber. Thus, a larger brake chamber, and consequently a longer and heavier chamber, undesirably increases the bending moment and stress on both the chamber extremity and the associated underside support structure of the vehicle. This problem is aggravated by the severe jolts and other very stressful forces typically encountered by heavy duty vehicles traveling at high speeds on rough road surfaces.
Another problem with the powerful compressed springs in such spring brake chambers is the fact that after the springs are allowed to expand within the confines of the chamber so as to apply the brakes, usually in a parking mode, the spring is still highly compressed and typically exerts a force within the chamber of as much as 1,000-1,200 pounds or more. Accordingly, if the chamber is inadvertently opened by a driver or service man without properly caging or otherwise securing the still-compressed powerful spring, damage and personal injury can result.
This problem is recognized and addressed by certain of the prior art patents. These include, for example, U.S. Pat. Nos. 4,565,120, 4,960,036 (and subsequent patents based thereon) and 5,105,727 (and subsequent reissue application Ser. No. 07/966,094 based thereon, now abandoned).
In the case of U.S. Pat. No. 4,565,120, a safety restraint is employed for such purposes; but it permits only partial separation of the bolted-together flanges, adds dimensionally to the chamber, and increases both the number of parts and manufacturing costs, as well as inconvenience of disassembly.
In the case of U.S. Pat. No. 4,960,036 (and subsequent patents based thereon) convenient disassembly in the field is intentionally inhibited or foreclosed, thereby limiting convenient internal access for inspection, repair or the like. In addition, even disassembly at a repair facility requires destructive deformation of the chamber sealing means and a significant level of care which through negligence or inadvertence may not always be present. A safe design not necessarily dependent upon such inconvenient and costly destructive approach for achieving safety is preferred.
In the case of U.S. Pat. No. 5,105,727, certain of the prior art shortcomings are avoided but the solution provided is devoid of a recognition of still other shortcomings. Thus, for example, no recognition or consideration is given to the desirability of maximizing the effective brake actuator stroke while limiting the brake chamber size and weight so as to accommodate limited installation spaces and minimize the aforementioned bending movements.
Another shortcoming of certain prior art assemblies is the need for and cost of additional machining required to provide a bearing surface for the reciprocating push rod where it sealingly passes through the common chamber wall of the centerbody into the service brake chamber and toward the actuator arm. This problem is encountered when employing, for example, a cast metal centerbody, a popular embodiment.
Again, to minimize the overall axial length of the chamber, at least a portion of the aforementioned bearing surface should be axially coincident with, not additive to, the common chamber wall, as will become apparent. Similarly, the diaphragm of the spring brake chamber should be disposed so that its thickness is axially-coincident with other requisite chamber components and thus not a necessary contributor to overall axial length.
Still another shortcoming of many prior art spring brake actuators is the fact that the outer circumferential periphery of the spring brake diaphragm is sealed by compression in an axial direction. Thus, the axial force of the power spring is undesirably oriented to have a tendency to unseal the diaphragm.